The invention is related to the field of photochromic materials and photomechanical transducers. Photochromic materials reversibly change their absorption spectrum by absorption of light. They are therefore used for security-printing, sunglass coatings or for optical data storage. A known photochromic material is bacteriorhodopsin which undergoes isomerization under illumination by photons with a quantum efficiency of 70%. The bacteriorhodopsin thereby switches between two states and the backswitching is induceable by thermal energy or photo-illumination with light with a different wavelength. There exist various classes of photochromic materials differing by their switching mechanism, such as H-tautomerism, dissociation, dimerization, cis-trans isomerization or charge transfer.
A photochromic material is known from U.S. Pat. No. 5,382,78. The disclosed material changes its photoelectric current value responding to the intensity of light irradiation at a constant temperature. Other materials disclosed in this publication exhibit a dependence of temperature.
U.S. Pat. No. 4,957,725 describes a method to form a layer of vanadium dioxide as a photosensitive layer. Vanadium dioxide changes its optical transmission, electrical resistance and spectral transmission characteristic depending on the wavelength of incident light.
A photomechanical transducer responding to light is already known from U.S. Pat. No. 4,252,440. The transducer comprises an ultrathin strip of polymeric or metallic film which is held under a small and constant strain. This film is sensitive to light and changes its strain due to thermal expansion or contraction in response to light absorption by the strip. Another light sensitive device is described in U.S. Pat. No. 4,891,512. A fiber end forming a cantilever is bent by an attached metallic pad which responds to the local heating produced by incident light.
In U.S. Pat. No. 4,945,514 is described a method of bistable optical information storage. The underlying principle is a light-activated shift of a field-dependent transition between two phases. The device is sensitive to near-UV- and visible light.
U.S. Pat. No. 5,242,491 discloses a gel whose volume changes by irradiation with light. The gel comprises a polymerized monomer, a photosensitive component and a liquid medium. The mixture changes its internal structure in that the contained polymer chains change their orientation with respect to each other. This intermolecular change involves the variation of the solvent in the polymer mixture thus varying the swelling factor of the gel. The interaction between the various molecules can however not be well defined on a macroscopic scale.